1. Field of the Invention
The present invention relates to a catadioptric projection objective for imaging a pattern provided in an object surface of the projection objective onto an image surface of the projection objective. The projection objective may be used for microlithography projection exposure machines. The invention relates, in particular, to exposure machines for semiconductor structures which are designed for immersion operation in an aperture range where the image-side numerical aperture NA is greater than 1.0.
2. Description of the Related Art
In the case of reducing optical imaging, in particular in the field of projection lithography, the image-side numerical aperture NA is limited by the refractive index of the surrounding medium in image space adjacent to the image surface of the projection objective. In immersion lithography the theoretically possible numerical aperture NA is limited by the refractive index of the immersion medium.
The immersion medium can be a liquid or a solid. An immersion liquid is disposed between an exit surface of the projection objective and the surface of the substrate to be exposed, which is arranged in the image surface. In contact-free solid immersion a planar exit surface of the projection objective is arranged at a working distance smaller than the operating wavelength to the substrate to be exposed such that evanescent fields emerging from the exit surface can be used for imaging (near-field lithography). Solid immersion with touching contact between the exit surface of the projection objective and the substrate is also possible.
The theoretical limit for image-side numerical aperture is normally not reached, since the propagation angles between the rays limiting the beam bundle and the optical axis then become very large. As a rule, NA should not substantially exceed approximately 95% of the refractive index of the last medium on the image side. For 193 nm, this corresponds to a numerical aperture of NA=1.35 in the case of water (nH2O=1.43) as immersion medium.
With immersion liquids whose refractive index is higher than that of the material of the last optical element with refractive power (also denoted last lens), or in the case of solid immersion, the refractive index of the material of the last lens (i.e. the last optical element of the projection objective adjacent to the image surface) acts as a limitation if the design of the exit surface of the projection objective is to be planar or only weakly curved. The planar design is advantageous, for example, for measuring the distance between wafer and objective, for hydrodynamic behaviour of the immersion medium between the wafer to be exposed and the exit surface of the projection objective, and for their cleaning. A further advantage of the planar design of the exit surface is that changes of the refractive index of the immersion liquid have little influence on the image quality. Such changes of the refractive index can be caused by changes of temperature, for example.
The exit surface must be of planar design for solid immersion, in particular, in order to expose the wafer, which is likewise planar.
For DUV (deep ultraviolet, operating wavelength of 248 nm or 193 nm), the materials normally used for the last optical element are fused silica (synthetic quartz glass, SiO2) with a refractive index of nSiO2=1.56 at 193 nm or CaF2 with a refractive index of nCaF2=1.50 at 193 nm. Given the limitations mentioned above, a numerical aperture of approximately NA=1.425 (95% of n=1.5) might be achieved if calcium fluoride is used for the last optical element. Using fused silica instead would allow numerical apertures of NA=1.48 (corresponding to approximately 95% of the refractive index of quartz at 193 nm). The relationships are similar at 248 nm.
It is contemplated that projection objectives having NA values in the range between about 1.35 and about 1.50 will become desirable in the near future. High NA values in this range and above can be obtained, for example, if at least one optical element in the projection objective is a high-index optical element made from a high-index material with a refractive index higher than that of fused silica, for example with n≧1.6 at the operating wavelength. For example, the high-index material may be sapphire which forms at least partly the last refractive optical element of the projection objective. Examples are shown in U.S. patent application with Ser. No. 11/151,465 and title: “Projection objective having high aperture and planar end surface” filed on Jun. 14, 2005 by the applicant. However, high-index materials in an optical quality suitable for this purpose are in limited supply and procedures for reproducably treating such materials during manufacturing are still being developed. Therefore it would be desirable to be able to produce very high NA projection objectives using only lenses made of established materials, such as fused silica. If, for example, a last optical element of a projection objective would be made of fused silica with nSIO2=1.56 at 193 nm an increase in image-side numerical aperture towards the limit value NA=1.56 requires that very high propagation angles α are present in the last optical element. This is demonstrated by table A where the image side numerical aperture NA is listed together with the propagation angle α between marginal rays and the surface normal to the planar exit surface of the projection objective (in most cases equal to half the opening angle of a beam bundle within the last optical element), and the respective sine of that maximum propagation angle α, which is the corresponding aperture sin α.
TABLE ANAα [°]Aperture sin α1.3559.90.8651.4063.80.8971.4568.30.9291.5074.00.961
It is difficult to control very high aperture values in the region of sin α≧0.8 or sin α≧0.9 with regard to optical correction. Since the outer marginal rays impinge at very large angles, small angular deviations lead to large offsets between an ideal image point and an actual image point with regard to geometrical lateral offsets. The geometrical optical aberrations as well as the aberrations of the wavefront have to be kept very low to obtain sufficient imaging fidelity.
The correction of chromatic aberrations (color correction) is an other problem in systems designed for wavelengths below about 260 nm since the Abbe numbers of those transparent materials that are available lie rather close to one another.
Further, in lithography, a flat (planar) image is essential to expose planar substrates, such as semiconductor wafers. However, generally the image surface of an optical system is curved, and the degree of curvature is determined by the Petzval sum. The correction of the Petzval sum is becoming more important in view of the increasing demands to project large object fields on planar surfaces with increased resolution.
One approach for obtaining a flat image surface and good color-correction is the use of catadioptric systems, which combine both refracting elements, such as lenses, and reflecting elements, such as mirror, preferably including at least one concave mirror. While the contributions of positive-powered and negative-powered lenses in an optical system to overall power, surface curvature and chromatic aberrations are opposite to each other, a concave mirror has positive power like a positive-powered lens, but the opposite effect on surface curvature without contributing to chromatic aberrations.
Further, the high prices of the materials involved and limited availability of crystalline calcium fluoride in sizes large enough for fabricating large lenses represent problems. It is known that, for a given size of the image field, the lens diameters generally increase as NA increases. This is partly due to the moderate refractive indices available for lenses in positions having large aperture values, particularly close to the image surface. However, large lens diameters are not desirable due to the limited availability of optical material in sufficient quality and also because the mechanical stability is negatively affected and the optical systems tend to become axially large. One means for reducing lens diameters would be to use large local concentrations of refractive powers. However, it is known that the contribution of lenses to aberrations roughly scales with the refractive power such that the aberration contributions increase as the refractive power increases.
Measures that will allow reducing the number and sizes of lenses and simultaneously contribute to maintaining, or even improving, imaging fidelity are thus desired.
In recent years, a number of catadioptric projection objectives have been proposed having a first, refractive imaging objective part for imaging the pattern provided in the object plane into a first intermediate image, a second, catoptic or catadioptric imaging objective part for imaging the first intermediate image into a second intermediate image, and a third refractive imaging objective part for imaging the second intermediate image directly onto the image plane. In a notation where “R” denotes a refractive imaging objective part, “C” denotes a catadioptric or catoptric objective part and “−” denotes an intermediate image, this type is briefly denoted as “R-C-R”. The first refractive objective part may be designed to obtain a suitable position, shape and correction status of the first intermediate image. The second objective part typically includes at least one concave mirror and may be designed to contribute substantially to Petzval sum correction. A primary task of the third, refractive objective part is to provide the high image-side numerical aperture and to correct aberrations associated therewith, particularly spherical aberration and coma. The terms “subsystem” and “objective part” will alternatively be used in this specification to denote a number of subsequent optical elements of the projection objective effective, in combination, as an “imaging system” for imaging a field into an optically conjugate field
US 2005/0190435 discloses catadioptric projection objectives having very high NA and suitable for immersion lithography at NA>1 with maximum values NA=1.2. The projection objectives comprise: a first refractive objective part for imaging the pattern provided in the object plane into a first intermediate image, a second objective part for imaging the first intermediate image into a second intermediate image, and a third refractive objective part for imaging the second intermediate image directly onto the image plane. The second objective part includes a first concave mirror having a first continuous mirror surface and a second concave mirror having a second continuous mirror surface, the concave mirror faces facing each other and defining an intermirror space. All concave mirrors are positioned optically remote from pupil surfaces. The system has potential for very high numerical apertures at moderate lens mass consumption. The full disclosure of this document and the priority documents thereof is incorporated into the present application by reference.
Catadioptric projection objectives including a catadioptric imaging objective part having one single concave mirror and arranged between an entry side and an exit side refractive imaging objective part are disclosed, for example, in U.S. application with Ser. No. 60/573,533 filed on May 17, 2004 by the applicant. Other examples of single-mirror R-C-R-systems are shown in US 2003/0011755, WO 03/036361, WO 2004/019128 or US 2003/0197946.
U.S. Pat. No. 6,600,608 B1 discloses a catadioptric projection objective having a first, purely refractive objective part for imaging a pattern arranged in the object plane of the projection objective into a first intermediate image, a second objective part for imaging the first intermediate image into a second intermediate image and a third objective part for imaging the second intermediate image directly, that is without a further intermediate image, onto the image plane. The second objective part is a catadioptric or catoptric objective part having a first concave mirror with a central bore and a second concave mirror with a central bore, the concave mirrors having the mirror faces facing each other and defining an intermirror space or catadioptric cavity in between. The first intermediate image is formed within the central bore of the concave mirror next to the object plane, whereas the second intermediate image is formed within the central bore of the concave mirror next to the object plane. The objective has axial symmetry and provides good color correction axially and laterally. However, since the reflecting surfaces of the concave mirrors are interrupted at the bores, the pupil of the system is obscured.
The Patent EP 1 069 448 B1 discloses another catadioptric projection objective having two concave mirrors facing each other. The concave mirrors are part of a first catadioptric objective part imaging the object onto an intermediate image positioned adjacent to a concave mirror. This is the only intermediate image, which is imaged to the image plane by a second, purely refractive objective part. The object as well as the image of the catadioptric imaging system are positioned outside the intermirror space defined by the mirrors facing each other. Similar systems having two concave mirrors, a common straight optical axis and one intermediate image formed by a catadioptric imaging system and positioned besides one of the concave mirrors are disclosed in Japanese patent application JP 2002208551 A and US patent application US 2002/0024741 A1.
European patent application EP 1 336 887 (corresponding to US 2004/0130806 A1) discloses catadioptric projection objectives having one common straight optical axis and, in that sequence, a first catadioptric objective part for creating a first intermediate image, a second catadioptric objective part for creating a second intermediate image from the first intermediate image, and a refractive third objective part forming the image from the second intermediate image. Each catadioptric system has two concave mirrors facing each other. The intermediate images lie outside the intermirror spaces defined by the concave mirrors. Concave mirrors are positioned optically near to pupil surfaces closer to pupil surfaces than to the intermediate images of the projection objectives.
International Patent application WO 2004/107011 A1 discloses catadioptric projection objectives having one common straight optical axis and two or more intermediate images which are suitable for immersion lithography with numerical apertures up to NA=1.2. At least one concave mirror is positioned optically near to a pupil surface closer to that pupil surface than to an intermediate images of the projection objective.
In the article “Nikon Projection Lens Update” by T. Matsuyama, T. Ishiyama and Y. Ohmura, presented by B. W. Smith in: Optical Micro-lithography XVII, Proc. of SPIE 5377.65 (2004) a design example of a catadioptric projection lens is shown, which is a combination of a conventional dioptric DUV system and a 6-mirror EUV catoptric system inserted between lens groups of the DUV system. A first intermediate image is formed behind the third mirror of the catoptric (purely reflective) group upstream of a convex mirror. The second intermediate image is formed by a purely reflective (catoptric) second objective part. The third objective part is purely refractive featuring negative refractive power at a waist of minimum beam diameter within the third objective part for Petzval sum correction.